High porosity, low tortuosity, variable-pore-size structured topology for capillary wicks

ABSTRACT

A heat pipe includes a a heat pipe wick having a gyroid or other periodic lattice structure, thereby providing relatively high porosity, high surface area, consistent pore size, structural stability, low tortuosity, and low convective acceleration.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/824,693 filed Mar. 27, 2019, which is hereby incorporated herein by reference.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Technology Transfer, US Naval Research Laboratory, Code 1004, Washington, D.C. 20375, USA; +1.202.767.7230; techtran@nrl.navy.mil, referencing NC 110707.

FIELD OF INVENTION

The present invention relates generally to capillary wicks, and more particularly to variable-pore-size topology for capillary wicks.

BACKGROUND

Advancements in small micro-electronic devices have increased power and power density resulting in new challenges in thermal management. Next-generation electronics cooling techniques better-able to manage waste heat would provide an avenue to meet the ever-increasing demands on computing performance.

Heat pipes can transport 10²-10⁴× as much heat for the same temperature gradient as solid conduction heat sinks, but they are limited by the pressure drop induced by the small flow channel sizes. The development of higher-performance heat pipes through the use of higher-performance wick geometries represent a crucial development.

Conventionally, wicks are constructed using two methods. The first method is a powder sintering process where the porosity is dictated by the particle sizing. The second method uses a fine mesh wound around a cylindrical core to provide the structured porosity. These systems can be augmented by secondary sintering of powders but do not allow arbitrary variation of the porosity throughout the volume. Generally, these methods only deliver a single axis of variable porosity: a gradient along the long axis of a cylinder for example. A conventional method of creating the grading is by stacking layers of differently-sized powders, then sintering the whole group at once. Previous efforts at structured geometries like trusses have also been demonstrated. Others have discussed the use of truss structures for wicks using porous materials. These previous efforts have explored the creation of truss structures through a laser-melting process, but not the variation of porosity on a macro/microstructured scale.

Tunable variable porosity provides significant performance benefits relative to constant or single axis graded porosity systems. This concept involves generating self-supporting porous structures. These structures provide benefits by reducing the complexity of the fluid path and mitigating the overall fluid losses inherent to the structure. The use of multi-scale porosity is also rarely seen in literature. Taking advantage of surface roughness on a meso-scale truss structure can increase effective capillary pressure by increasing the effective surface area.

SUMMARY OF INVENTION

A performance limitation of structured wicks is the feature-size or wall-size limit. Every manufacturing method makes a trade-off between minimal manufacturable feature size and wall thickness. For a working fluid that operates optimally when paired with larger porosities, the manufacturing limit of the wall thickness will have minimal effect. For working fluids that require smaller porosities, the wall thickness becomes a more significant constraint, particularly in light of the fact that the only way to currently achieve extremely small porosities is with sintered powder, which produces a structure with a very low porosity. Given high performance manufacturing capability the present invention should be able to have high performance across the full range of working fluids. As the manufacturing quality (minimum feature size) reduces, increasing your wall size, the working fluids that will generate the highest performance will require larger porosities.

Patents on this general subject suggest the same general idea: using several mesh sizes along the length of a heat pipe to roughly grade the porosity axially. Alternately, different grain-sizes of powder may be layered to create a sintered wick with regions of differently-sized pores.

The similarity between the identified existing approaches is the limitation of conventional manufacturing. It is tremendously difficult to vary the properties of a wick spatially. The present invention includes a new method that lends itself to a variety of fabrication methods including additive manufacturing that would enable 3-axis variability and the local optimization of pore size, porosity, tortuosity, convective acceleration, structural rigidity, and robustness to vibration and impulse.

According to one aspect of the invention, a heat pipe includes a heat pipe wick having a gyroid or other periodic lattice structure, thereby providing relatively high porosity, high surface area, consistent pore size, structural stability, low tortuosity, and low convective acceleration.

Optionally, the heat pipe wick includes a finite-order fractal cross-section, thereby providing relatively high surface area and porosity for a given channel size.

Optionally, the heat pipe wick comprises variable pore sizes.

Optionally, the variable pore sizes are produced via scaling a basic geometry of the heat pipe wick.

Optionally, the variable pore sizes are produced via skewing a basic geometry of the heat pipe wick.

Optionally, the variable pore sizes are produced via implementing a basic geometry of the heat pipe wick on multiple scales in the same bulk structure.

Optionally, the heat pipe is integrated into a hypersonic leading edge.

Optionally, the heat pipe is a loop heat pipe.

Optionally, the heat pipe is integrated into a vapor chamber heat spreader.

Optionally, the heat pipe wick includes local fractal or high surface roughness to increase capillary force, and wherein the local fractal or high surface roughness is configured to reduce overall pressure drop, thereby greatly improving heat capacity.

The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows exemplary gyroid geometry with solid walls.

FIG. 2 shows exemplary stretched gyroid geometry enabling continuously-variable porosity capabilities.

FIG. 3 shows exemplary cross sections for an 8th-order Quadratic Flake and a 5th-order Quadratic Cross (top), and Koch Snowflakes of order zero to 4. Each illustrates structured geometry with a large perimeter relative to the enclosed area.

FIG. 4 shows porosity hierarchy with partially sintered porous powder, unstructured porosity, controllable gradient porosity, structured porosity, tunable structured porosity and then macro micro porosity.

FIG. 5 shows other exemplary periodic high-surface-area lattices derived from zero potential surfaces or minimal surfaces.

FIG. 6 shows an exemplary leading edge heat pipe with additively manufactured variable wick structure using truss or directly printed variable porous wick.

DETAILED DESCRIPTION

A 3-axis variable-porosity-wick heat pipe can be leveraged to improve the effective capillary pumping pressure and minimize hydraulic losses. By using self-supporting structures such as the gyroid, truss structures, or other structured topology, a structure with continuously and arbitrarily varying porosity and pore size throughout a volume can be generated. This enables total control of the overall porosity resulting in improved peak heat transfer and more robust heat pipes. Previous work has identified methods to enable radial or axial variations in the porosity of the wick structure through variable powder sizing in a sintered wick process. These previous processes are based on powder size variation and limit the directions and degree to which a gradient can be produced as well as the accuracy with which an arbitrary gradient can be represented.

The goal is to maximize heat transfer in a two-phase heat transfer system such as a heat pipe or loop heat pipe. In order to do so, we can increase the capillary pressure with small pore sizes, increase the bulk porosity of the wicking structure, increase the wetted surface area of the wick structure, reduce the tortuosity of the flow paths, reduce the convective acceleration of the flow paths, use gradients in pore sizes where large or smaller flow paths are locally preferred, and construct the wick with multi-scale flow channels. The heat transfer capability of a heat pipe is directly related to the capillary pressure rise available and the pressure losses within the system. Small pore sizes increases capillary pressure, but also increases viscous pressure losses. Increasing the bulk porosity enables more efficient cross-sectional fluid flow. Increasing the wetted surface area increases the capillary pressure produced. Reducing the tortuosity of the flow paths reduces pressure losses. Reducing the convective acceleration of the flow paths reduces pressure losses. Graded pore sizes allow the creation of high capillary pressure where needed without incurring small-channel pressure losses everywhere. Multi-scale flow channels enable low-flow-rate high-pressure channels in parallel with high-flow-rate low-pressure channels. The consideration and improvement of each of these features will increase the heat transport capacity, temperature range, robustness, and reliability of a two-phase heat transfer system.

Sintered powder wicks have excellent pore size but poor porosity, tortuosity, convective acceleration, structural rigidity, and robustness to vibration and impulse. Mesh screen wicks have poor pore size, tortuosity, and convective acceleration, but have excellent structural rigidity and robustness.

A critical capability of the present invention is the ability to continuously and widely vary the porosity of a coherent, structured topology throughout a volume. The purest expression of this idea is the use of a mesostructure, such as the gyroid unit cell shown in FIG. 1, to directly define the geometry of the wick. A given section can then be deformed arbitrarily, shown in FIG. 2, to produce variations in porosity along any dimension.

The gyroid is one example of the proposed mesostructural geometry concept. The gyroid provides several benefits compared to other structures. For the purposes of wicking, it provides a favorable ratio of wetted perimeter to void fraction while also allowing fluid to progress in any Cartesian direction through the lattice due to its interconnected channels. The channels in the gyroid provide simple paths with low tortuosity and low convective acceleration. For the purposes of manufacture, the gyroid has very limited overhanging areas, is fully connected, and an absence of sharp features. This makes the geometry favorable for additive manufacturing—particularly at a small scale. It also provides excellent load paths for stress relieving in all directions and retains structural rigidity.

The microstructural geometry is an additional expression of tunable porosity. The lattice mesostructure can itself be made of porous material, such as being formed from sintered powder. In this case, the degree of sintering of the powder directly affects the characteristic diameter of the pores. This allows for independent control of mesostructural and microstructural porosity: that is, the meso-scale porosity resulting from the space between lattice walls, and the micro-scale porosity within the partially-sintered lattice walls.

By varying pore size across the geometry, improvements can be made. In particular, small pore size gives high capillary pressure, but high pressure drop. This would be desirable desirable near heat source & along vapor-liquid interface locations. Accordingly, large pore size gives low capillary pressure, but low pressure drop, and this would be desirable elsewhere.

Additional useful geometries include finite-order fractal topologies in two and three dimensions, such as the Koch Snowflake and the Quadratic Flake or Cross. These are illustrated in FIG. 3 and feature large perimeters for a given enclosed area.

In summary, four main features are being described herein. First, we have identified the gyroid and other periodic lattice structures for use as heat pipe wicks because of their capability to deliver high porosity, high surface area, consistent pore size, structural stability, additive manufacturability, low tortuosity, and low convective acceleration. As a subset of the three-dimensional topologies, we have identified the advantages of finite-order fractals as two-dimensional channel cross-sections that produce high surface area and porosity for a given channel size.

Second, we propose to use these structures to deliver tunable and variable pore sizes by scaling, skewing, and distorting the basic geometry. This may optionally include implementing such geometries and channels on multiple scales in the same bulk structure.

Third, we propose to use these variable-pore-size heat pipes conformal to and integrated into a hypersonic leading edge in addition to the wick's use in traditional cylindrical heat pipes, loop heat pipes, and vapor chamber heat spreaders.

Fourth, we propose that the intentional addition of fractal or high surface roughness locally can increase the capillary force. By defining explicitly the locations to introduce the additional fractal roughness the impact to overall pressure drop can be reduced to greatly improve the heat capacity of the proposed wick design.

Of the porosity generating methods shown in FIG. 4, only three exist in literature: micro-scale porosity resulting from uniformly sintered powder, meso-scale unstructured porosity resulting from burnout or dissolution of a filler material in a packed bed, and variable micro-scale porosity resulting from discrete regions of differently sized powders or mesh screens stacked together.

Novel contributions described herein include the remaining three geometries proposed: meso-scale structured porosity resulting from selective coalescence of material through advanced manufacturing processes, tunable meso-scale structured porosity that continuously grades the characteristic pore diameter of a lattice throughout a volume, and multi-scale porosity that leverages both tunable structured porosity on the meso-scale and independently grades the micro-scale porosity of the lattice walls themselves by varying the degree of coalescence used to form them.

The novel geometric configurations for the structured porosity described herein include gyroids and other zero-potential surfaces or minimal surfaces as well as finite-order fractal cross-sections. The family of zero-potential surfaces, some seen in FIG. 5, provide benefit through smooth surfaces reducing localized stresses and the pressure loss associated with flow through those regions. These structures introduce a new capability that generates the next generation of high-performance heat pipes for a variety of applications.

The novel use of these advanced systems in the application of a leading edge hypersonic vehicle as seen in FIG. 6 is also contemplated. The temperature regimes associated with hypersonics results in working fluids that lend themselves to large-pore structures. The high-temperature concentrated heat loads in hypersonics is an ideal application of these high-performance heat transfer systems and this enabling technology.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application. 

What is claimed is:
 1. A heat pipe comprising: a heat pipe wick having a gyroid or other periodic lattice structure, thereby providing relatively high porosity, high surface area, consistent pore size, structural stability, low tortuosity, and low convective acceleration.
 2. The heat pipe of claim 1, wherein the heat pipe wick includes a finite-order fractal cross-section, thereby providing relatively high surface area and porosity for a given channel size.
 3. The heat pipe of claim 1, wherein the heat pipe wick comprises variable pore sizes.
 4. The heat pipe of claim 3, wherein the variable pore sizes are produced via scaling a basic geometry of the heat pipe wick.
 5. The heat pipe of claim 3, wherein the variable pore sizes are produced via skewing a basic geometry of the heat pipe wick.
 6. The heat pipe of claim 3, wherein the variable pore sizes are produced via implementing a basic geometry of the heat pipe wick on multiple scales in the same bulk structure. The heat pipe of claim 1, integrated into a hypersonic leading edge.
 8. The heat pipe of claim 1, wherein the heat pipe is a loop heat pipe.
 9. The heat pipe of claim 1, wherein the heat pipe is integrated into a vapor chamber heat spreader.
 10. The heat pipe of claim 1, wherein the heat pipe wick includes local fractal or high surface roughness to increase capillary force, and wherein the local fractal or high surface roughness is configured to reduce overall pressure drop, thereby greatly improving heat capacity. 